This close-up of an Octagonal case-bearer Caterpillar shows the fine detail of the structure of case. My suspicion is that the little black rods may be frass (a.k.a. "poop") that are held together with silk spun from the caterpillar's mouth. Look at how perfectly the rods are aligned! Isn't nature amazing?! It is easy to see where the caterpillar stopped and later started new sections of the tube.
***Update: I later learned that my suspicion was correct, the little black rods are frass!